Module state access from extension methods

There was a conversation about this matter on this list some while
ago. There should be people here that remember it, so I am sending it
here before sending it to python-ideas.

I have a PEP draft prepared for allowing type methods in C extensions
access to per-module state. This should solve some problems of PEP 489
multiphase initialization.

conversation - https://mail.python.org/pipermail/import-sig/2015-July/001022.html
PEP - https://github.com/Traceur759/pep-drafts/blob/master/pep-C.rst
Implementation - https://github.com/Traceur759/cpython/pull/4